When an impedance of a signal source in an electrical signal is high, an input impedance of an amplifier must be considerably high to measure an electrical signal without loss. High input impedance amplifiers have been developed for signal measurement in an electrical signal measurement field. In the case that the amplifier is an operational amplifier, an input impedance of a sensor circuit is nearly decided by a self input impedance of the operational amplifier. Especially, because an input impedance of an operational amplifier used in electrical signal measurement is an input impedance of a sensor circuit, other circuit elements are almost not available as impedance rising elements. In conventional electrical signal measurement, an electrical signal can be sufficiently measured only with a self input impedance of an amplifier without loss.
In bioelectrical signal measurement, an electrolyte-containing metal electrode is attached to the skin of an examinee to measure an electrical signal. The electrolyte electrode attached to the skin of an examinee may cause the examinee to feel displeasure and inconvenience of a measurement procedure. Inconvenience of a measurement procedure and the examinee's displeasure serve as an obstacle to drastic demand for utilization of a bioelectrical signal.